Generally the invention relates to the control of a switch on the primary side of a switched-mode power supply. Particularly the invention relates to generating switching pulses, which are supplied to the switch element on the primary side of a freely oscillating switched-mode power supply, so that the primary side utilises a rectified image voltage, which represents the behaviour of the voltage on the secondary side.
In a known way a switched-mode power supply comprises a transformer which divides the power supply into primary and secondary sides. The primary side contains a switch element which at a certain frequency interrupts the current flowing through the primary winding of the transformer according to a certain duty cycle. Usually the switch element is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). As a switch element its operation is controlled by switching pulses, which are supplied to the gate of the MOSFET. The duty cycle of the switching pulses determines the momentary electric power supplied to the transformer. The frequency of the switching pulses is of importance mainly regarding dimensioning of the components in the switched-mode power supply.
In order to generate the switching pulses it is possible to use an oscillator with a fixed frequency, or a particular PWM controller (Pulse Width Modulation). However, in many cases it is advantageous to use a freely oscillating circuit on the primary side for generating the switching pulses, so that the frequency and the duty cycle of the circuit are adapted to the operating conditions of the switched-mode power supply. In generating switching pulses the object is to have the primary side to supply at each moment, controlled by the pulses, the optimal electric power to the secondary side through the transformer. The secondary side often contains measurement circuits in order to supervise the secondary current and/or the secondary voltage. The information provided by them is returned to the primary side, for instance via an opto-isolator, because safety aspects require galvanic isolation between the primary and secondary sides. For instance, if the measurement on the secondary side shows that the secondary voltage increases over its usual value, then the feedback through the opto-isolator to the switching pulse generating circuit reduces the duty cycle, and thus it reduces the amount of electric power supplied to the transformer.
It is known as such to implement the switched-mode power supply also as a so-called primary-controlled supply, i.e. without any feedback from the secondary side to the primary side. For instance, the patent publication U.S. Pat. No. 4,172,276 presents a switched-mode power supply which in addition to the primary and secondary windings of the isolating transformer has a third winding generating a certain auxiliary voltage on the primary side. The switching pulses controlling the switch transistor, which interrupts the primary current, are controlled by a reference voltage generated by certain capacitors and a zener diode, whereby a problem is particularly that the potential difference over the zener diode depends on the current passing through it. As one possibility the text presents to connect a constant current supply in parallel with the zener diode, but the actual invention presented in the publication is based on the fact that in order to generate a corrected reference voltage a compensating voltage is added to the voltage over the zener diode, so that the compensating voltage is generated according to the measured primary current. Thus there is a current measurement circuit on the primary side. On the other hand they also use an additional winding for detecting the waveform of the secondary voltage and to imitate it on the primary side. The original reference voltage which supplies current via the zener diode to the switch transistor base is generated just with the aid of this additional winding.
From the Finnish patent application no. FI-952571 there is known a coupling which controls the output voltage and current of a switched-mode power supply in a very versatile and accurate manner on the basis of only the functions of the couplings on the primary side. In the solution according to the publication the auxiliary winding in the isolation transformer generates an image voltage whose waveform corresponds to that of the secondary voltage. The image voltage is corrected with different correction terms, which compensate for its distortion as the output characteristics of the switched-mode power supply vary. The duty cycle of the switching pulses of the switch transistor is determined on the basis of the corrected image voltage. The switched-mode power supply according to the publication is intended to be used particularly in battery chargers, where the output current must remain almost constant as the output voltage rises, from a value corresponding to the terminal voltage of a completely discharged battery, to a value corresponding to the terminal voltage of a fully charged battery, and the output voltage must then remain below a certain maximum value, even if the fully charged battery then would not receive any essential current. The coupling according to the publication meets well the objectives placed on it, but regarding its structure it is rather complicated, which increases the manufacturing costs.
From the Finnish patent application no. FI-970063 there is known a primary-controlled switched-mode power supply according to FIG. 1, which in a normal state supplies a constant power via the transformer M101 to the secondary side, and where the auxiliary winding (the winding connected to the resistor R110 and the capacitor C105) of the transformer M101 on the primary side generates an auxiliary voltage representing the secondary voltage. Central components of the freely oscillating circuit, which generates the switching pulses, are the capacitor C104 and the transistor Q102. During the periods when the switch transistor Q101 is non-conducting the main part of the energy stored in the magnetic field of the transformer M101 is discharged to the secondary side. A part of the energy is however discharged through the auxiliary winding. The diode D105 acts as a rectifier, which in the point A forms a negative potential smoothed by the capacitor C123, whereby the absolute value of the potential is proportional to the secondary voltage. The zener diodes D108 and D109 regulate the operation on the primary side controlled by this potential. The threshold level in the reverse direction of the zener diode D108 is the higher of these. As the secondary voltage tends to rise the potential of the point A is the more negative the higher the secondary voltage is. When the reverse voltage over the zener diode D108 exceeds the reverse threshold level of said zener diode, then the zener diode starts to conduct in the reverse direction, which pulls the emitter of the transistor Q102 in the more negative direction. Then even a very small current induced by the primary current in the current loop formed by the auxiliary winding and the resistors R108 and R112 is sufficient to bring the transistor Q102 into the conducting state, whereby the conducting periods of the switch transistor Q101 will be very short, and the amount of energy which the primary side supplies to the isolating transformer will decrease. On the other hand, when the secondary voltage again decreases, then the absolute value of the potential in the point A on the primary side will decrease. When the zener diode D108 doesn""t anymore conduct in the reverse direction, then the power supply will again operate according to the usual principle of constant power control.
If the secondary current continues to increase, then this will reduce the secondary voltage and the absolute value of the potential at the point A, proportional to the secondary voltage. As long as the point A is sufficiently negative in order to keep the zener diode D109 conducting in the reverse direction, then the potential at the base of the transistor Q103 remains low and the transistor Q103 is non-conducting. If the reverse direction voltage over the zener diode D109 falls below the reverse threshold voltage of the zener diode D109, then it stops conducting in the reverse direction, and a certain positive voltage appears via the resistor R111 at the base of the transistor Q103, whereby the transistor Q103 goes into the saturation state and forms a new current path between the transistor""s Q102 emitter and the negative terminal of the rectified input voltage. At low currents the collector-emitter voltage of a typical npn-transistor in the saturation state is only about 50 mV, i.e. quite low compared to the forward voltage over the diode D107, which is about 0.6 V. This increases the discharging of the charge at the gate of the switch transistor Q101 during each operating cycle, which reduces the amount of electric power supplied to the isolating transformer.
In secondary-controlled switched-mode power supplies using feedback from the secondary side via an opto-isolator or some other galvanic isolating component to the primary side, the operation of the circuit becomes a problem in a case when the feedback is not acting in the desired way, due to a fault either in the measurement circuits on the secondary side or in the actual feedback. It would be desirable that the circuit operates in a controlled manner also in a situation like this. The known couplings further have that characteristic feature of analogue electronics, that any circuit and function would be even more advantageously realised if it would be possible to further reduce the number of components, the space they occupy in the device, and/or their manufacturing costs.
The object of the present invention is to present a switched-mode power supply which is advantageous regarding the manufacturing techniques and which has a low number of components. A further object of the invention is to present a switched-mode power supply which operates reliably also in exceptional situations. Further an object is to present a switched-mode power supply having advantageous EMC characteristics (ElectroMagnetic Compatibility).
The objects of the invention are attained by forming on the primary side a rectified image voltage which represents the secondary voltage, and by using this rectified image voltage for the control of freely oscillating circuits generating the switching pulses.
The characteristic features of a switched-mode power supply according to the invention are the use of a rectifying and charge storing coupling between the terminals of an auxiliary winding in order to generate an image voltage, which on the primary side represents a certain voltage of the secondary side, and the use of a coupling from said rectifying and charge storing coupling to a freely oscillating circuit in order to control how switching pulses are generated on the basis of the value of said image voltage.
The switched-mode power supply according to the invention utilises in the isolating transformer an auxiliary winding in addition to the actual primary and secondary windings, whereby the auxiliary winding on one hand supplies the xe2x80x9cpumpingxe2x80x9d pulses required by the freely oscillating circuit generating the switching pulses, and whereby the auxiliary winding on the other hand is further connected to a coupling which rectifies and stores a charge on the primary side. The last mentioned coupling contains most advantageously a diode and a capacitor in series, which form a loop together with the auxiliary winding. In this loop an image voltage is generated over the capacitor, which voltage is proportional to that voltage which prevails between the output terminals of the secondary side.
The image voltage generated according to the invention can be utilised by different means for generating the switching pulses on the primary side. Common to these means is that changes in the image voltage act on the duty cycle and/or on the switching frequency in the same direction as would the corresponding changes measured in the secondary voltage.
In a simple overvoltage supervision the duty cycle is reduced with the aid of the image voltage in a situation where the secondary voltage tends to rise too much. The image voltage is coupled to a coupling which can be triggered by a threshold value, which coupling in the simplest case is a zener diode. If the image voltage exceeds the threshold value the last mentioned coupling begins to xe2x80x9cleakxe2x80x9d voltage to the oscillating circuit (or from this circuit), which forms the switching pulses. The potential change in the oscillating circuit resulting from such a leak reduces the duty cycle, which in turn reduces the amount of electric power supplied to the transformer and thus reduces the secondary voltage.
In a simple switched-mode power supply the overvoltage supervision realised with the aid of the image voltage can even be the only coupling which supervises the secondary voltage. In a feedback switched-mode power supply with secondary control it is also possible to secure a limited secondary voltage also in a fault situation when the measurement of the secondary voltage and/or the feedback of the measurement information to the primary side is not working, for some reason or other. In a securing arrangement of this kind the coupling which is triggered on the basis of the threshold value is connected in parallel with the phototransistor of the opto-isolator, whereby a limiting signal is obtained directly from the image voltage on the primary side if the opto-isolator is unable to deliver the limiting signal.
The use of the image voltage can be applied also in a secondary-controlled switched-mode power supply where the feedback has an xe2x80x9cinvertedxe2x80x9d purpose compared to the normal one. A normal feedback means such a feedback which generates a limiting signal, i.e. it informs when the supply of electric power to the transformer must be limited. In the inverted feedback control the secondary side xe2x80x9crequestsxe2x80x9d more electric power via the feedback, in other words, the feedback tells when the transformer should supply more electric power. Then the image voltage according to the invention is used to start the primary side, in other words to generate such switching pulses which cause electric power to be supplied to the secondary side even if the energy on the secondary side is not yet sufficient to use the transmitting component (the light-emitting diode, LED, of the opto-isolator) in the feedback.
As we studied the invention we also found a surprising aspect, according to which the connection of the auxiliary winding in the manner according to the invention substantially reduces such electromagnetic interference which is transmitted along the conductors to the mains network. The invention enables the switched-mode power supply to be constructed without a so called common mode choke in a filtering arrangement, whose task is to prevent radio frequency interference from propagating into the mains network, or without so called Y capacitors, which are normally used between the primary and secondary sides in order to reduce interference.